This invention relates to processes for the treatment of sludge generated by wastewater treatments, and more particularly to a process for chemically heating and drying sludge by which all pathogen and non pathogen microorganisms are eliminated, obtaining a product which can be used as a good quality fertilizer.
In the wastewater treatment processes, it is produced a great quantity of a bio-mass called sludge, normally comprising 98% of water and 2% of solids, which contains a great variety of microorganisms, such as bacteria, parasites and pathogen viruses among others.
Due to the biological content of the sludge, it is highly polluting for the environment and represent a serious health risk for the population.
The disposition of the sludge is a complex and problematic task, which is actually made using the following processes:
a) Land Fill
This process consists on confining the sludge in a field having the following characteristics: a low permeability, absence of geological faults, very low volumes of rainfall, and far away from towns and cities in order to avoid the pollution of the aquiferous layers with possible leaks of polluted liquid from the land fill.
Obviously, before the disposition of the sludge in the field, the surface of the field has to be sealed with a waterproof and flexible plastic film, and after the deposit of each sludge layer, it has to be applied a very thick soil layer.
Among some of the disadvantages of the above referred process are: its high investment cost, its high operation cost and its null economic profit.
b) Pasteurization Process and Compost Production
This process comprises the steps of mixing the sludge with pasture; heating the resultant mix with steam and fast cooling the heated mix in order to eliminate all the pathogen microorganisms contained in the sludge and obtain a fertilizer called Compost free of any bacteriological polluting agent.
However, this process is recommended for using in low capacity wastewater treatment plants, in order to apply the produced Compost to cultivation lands in the proximity of the treatment plant due to the high transportation costs and low selling price of the Compost which make unprofitable the transportation of the Compost to places very far away from the plant.
c) Rotary Dryers
The sludge drying process produces a good quality fertilizer having an average commercial value, due to its high organic compound and nutrient content such as nitrogen, phosphorous and potassium, which result from the elimination of a great percentage of the water contained in the sludge. This nutrient content can be raised to a total of 6.5% if expressed as N, P2O5 and K2O (the fertilizers by definition, must contain 20% minimum of such nutrients).
Even though that with the drying process it is obtained a product having an average commercial value and that by said process it is solved the bacteriological pollution problem, the fuel consume used to generate the heat needed by the drying process, represents a high cost. Therefore, said process requires a high investment cost and has a very high operation cost, which make the process unprofitable for treating low quantities of sludge.
d) Sludge Incineration
The sludge incineration process, has some advantages when compared with the sludge drying process, due to the fact that during the incineration, it is used the combustion heat of the organic matter contained in the sludge, thus decreasing the energy consumption and furthermore, the air volumes to be heated and moved are significantly smaller which constitute an additional energy consumption decrease.
The main disadvantages of the sludge incineration process are related with the drying of the sludge, mainly because of the following problems: the investment, operation and maintenance costs of the incineration step are higher than the costs of the drying step, furthermore, the incineration process is highly pollutant to the environment, because the gaseous effluents produced by the incinerator are very acidic, and contain very toxic metallic compounds such as lead, arsenic, cadmium and mercury (normally present in the urban wastewater), which are considered as very dangerous for the environment and for the health of humans and animals. Additionally, the disposition of the ashes generated by the incinerator and recollected in the ash separator, have the same polluting agents as the gaseous effluents and therefore the use of said ashes for the production of cement for example, is very restrained.
e) Sludge Drying Beds
Drying beds are the most widely used method of sludge dewatering in the United States. Sludge drying beds are typically used to dewater digested sludge. After drying, the sludge is removed and either disposed of in a land fill. The most commonly used type of dry bed is the sand drying beds, which are generally used for small and medium-sized communities. In a typical sand drying bed, sludge is placed on the bed in a 20 to 30 cm layer and allowed to dry. Although the principal advantage of the drying bed is its low cost, there are highly pollutant to the environment and the dry product can not be used due to its high content of polluting agents such as bacteria and viruses.
d) Lime Stabilization
In the lime stabilization process, lime is added to untreated sludge in sufficient quantity to raise the pH to 12 or higher. The pH creates an environment that is not conductive to the survival of microorganisms. Consequently, the sludge will not putrefy or create odors so long the as the pH is maintained at this level. However, lime stabilization does not destroy the organics necessary for bacterial growth. Therefore the sludge can not be used and must be treated with an excess of lime or disposed of before the pH drops significantly.
e) Ocean Disposal
The ocean disposal is highly pollutant and therefore less and less accepted by the governments.
e) Other Processes
Some investigators have studied and developed some other new processes by which there are obtained interesting results. For example by mixing the sludge with construction cement powder and processing the mix inside a granulation machine, it can be obtained granules of fertilizer useful for certain acidic soils such as those predominating in Europe or at the coasts of the Mexican Gulf.
In order to solve the above referred problems, applicants developed a process for the treatment of sludge which do not produce toxic residues and has low investment and operation costs.
The process according with the present invention comprises the steps of: mixing the sludge with sulphuric acid; feeding the mix to a granulator machine; adding sulphuric acid and anhydrous ammonium to the mix inside the granulator in order to produce an exothermic reaction between the sulphuric acid and the anhydrous ammonia which produces sufficient heat to evaporate the water contained in the sludge and to eliminate all the pathogen and non pathogen microorganisms; adjusting the amount of sulphuric acid and anhydrous ammonium that is added to the granulator in order to obtain a final dry product having a pH of between about 1.0 and 5.0 and having an humidity of 3.5% maximum by testing the acidity and humidity of a sample of the dry product at an exit outlet of the granulator; sieving the dry product in order to separate the product particles in three parts: a first part comprising particles having a normal size of 96% by weight of between 4 to 18 mesh screen, a second part comprising particles having a size below normal, and a third part comprising particles having a size above normal; and recovering the final product particles having normal size;
The process of the present invention, uses the heat produced by the reaction between the sulphuric acid and ammonia for the transformation of the sludge on a nitrogenous fertilizer having a minimum content of nitrogen of 18%, by which it is avoided the use of a burner and the related fuel consume.
The heat produced by the above referred reaction is sufficient for the evaporation of the water contained in the sludge and for the elimination of all the pathogen and non pathogen microorganism, producing a dry product having a low pH, free of polluting agents and a high commercial value.
Furthermore, the investment on installing a plant for the operation of the process of the present invention is lower than processes such as the sludge incineration processes which do not generates commercial products. And the investment cost is recoverable in function of the total sales of the fertilizer produced by the process of the present invention.
Finally, the process of the present invention can be used also to treat hen droppings and guano.
It is therefore a main objective of the present invention, to provide a process for the treatment of sludge generated by wastewater process treatments, which uses the heat produced by the reaction between the sulphuric acid and ammonium to evaporate the water of the sludge.
It is also a main objective of the present invention, to provide a process for the treatment of sludge of the above disclosed nature by which all the pathogen microorganisms contained in the sludge are eliminated.
It is another objective of the present invention, to provide a process for the treatment of sludge of the above disclosed nature, in which the investment and maintenance cost are lower than other processes for the treatment of sludge.
It is also a main objective of the present invention, to provide a process for the treatment of sludge of the above disclosed nature, which produce a fertilizer having a high commercial value.
It is still another objective of the present invention to provide a process for the treatment of sludge of the above disclosed nature, by which it is avoided the use of a burner and the related fuel consume.
It is still a main objective of the present invention, to provide a process for the treatment of sludge of the above disclosed nature, which do not produce toxic residues.
It is another objective of the present invention to provide a process for the treatment of sludge of the above disclosed nature, which can be used to treat also hen droppings and guano.
These and other objects and advantages of the, process for the treatment of sludge of the present invention will become apparent to those persons having an ordinary skill in the art, from the following detailed description of the invention.